An aircraft may include various types of rotating electrical machines such as, for example, generators, motors, and motor/generators. Motor/generators are used as starter-generators in some aircraft, since this type of rotating electrical machine may be operated in both a motor mode and a generator mode. A starter-generator may be used to start the engines or auxiliary power unit (APU) of an aircraft when operating as a motor, and to supply electrical power to the aircraft power distribution system when operating as a generator. Thus, when operating as a motor, a starter-generator may be designed to supply mechanical output torque sufficient to start the engines.
One particular type of aircraft starter-generator includes three separate brushless generators, namely, a permanent magnet generator (PMG), an exciter generator, and a main motor/generator. The PMG includes permanent magnets on its rotor. When the PMG rotor rotates, AC currents are induced in stator windings of the PMG. These AC currents are typically fed to a regulator or a control device, which in turn outputs a DC current if the starter-generator is operating in a generator mode. Conversely, if the starter-generator is operating in a motor mode, the control device supplies AC power.
If the starter-generator is operating in the generator mode, DC current from the regulator or control device is supplied to stator windings of the exciter. As the exciter rotor rotates, three phases of AC current are typically induced in the exciter rotor windings. Rectifier circuits that rotate with the exciter rotor rectify this three-phase AC current, and the resulting DC currents are provided to the rotor windings of the main motor/generator. Finally, as the main motor/generator rotor rotates, three phases of AC current are typically induced in the main motor/generator stator, and this three-phase AC output can then be provided to a load.
If the starter-generator is operating in the motor mode, AC power from the control device is supplied to the exciter stator. This AC power induces, via a transformer effect, an electromagnetic field in the exciter armature, whether the exciter rotor is stationary or rotating. The AC currents produced by this induced field are rectified by the rectifier circuits and supplied to the main motor/generator rotor, which produces a DC field in the rotor. Variable frequency AC power is supplied from the control device to the main motor/generator stator. This AC power produces a rotating magnetic field in the main stator, which causes the main rotor to rotate and supply mechanical output power.
The above-described starter-generator may include relatively complex and heavy power electronics circuits in the control device. For example, some control devices may include inverters, for converting DC to AC power, rectifiers, for converting AC power to DC power, and potentially complex voltage and frequency control circuits. Although brush-type DC machines may alleviate the need for some of these complex and heavy electronic circuits, these also suffer certain drawbacks. For example, the brushes tend to wear fairly quickly, reducing machine reliability, and increasing the need for periodic maintenance and cleaning.
One prior approach to addressing the above-mentioned drawbacks was developed by some of the inventors of the present invention. The prior approach, disclosed in U.S. patent application Ser. No. 10/______, entitled “Gas Turbine Engine Starter Generator with AC Generator and DC Motor Modes,” and assigned to the assignee of the instant application, provides a specially wound main stator that can be selectively configured as either a multi-pole AC stator or a multi-pole DC stator. Although this prior approach addresses the noted drawbacks, it too presents certain drawbacks. In particular, the specially wound main stator, which includes additional segments and windings, is more complex than a conventionally wound stator, which can increase overall costs, and adversely affects power quality when operating in the generator mode.
Hence, there is a need for a starter-generator that does not rely on relatively complex and heavy inverters and frequency control circuits for proper operation, and/or does not suffer reduced reliability from brush wear, and/or the need for potentially frequent maintenance and cleaning and/or does not use a specially wound stator. The present invention addresses one or more of these needs.